Alarm system responsive to sonic vibrations



D. R. MCCOY 2,832,915

, ALARM SYSTEM RESPONSIVE TO some VIBRATIONS April 29, 1958 Original Filed Feb. 3, 1950 AMPLIFIER POWER SUPPLY INVENTOR D. R. MCCOY HYDROPHONE ATTORNEY ject to tampering.

ALARM SYSTEM RESPONSIVE TO SONIC VIBRATIONS Donald R. McCoy, North Hollywood, Calif., assignor to Bendix Aviation Corporation, North Hollywood, Calif, a corporation of Delaware application February 3, 1950, Serial No. 142,248. vDivided and this application September 14, 1953, Se-

rial No. 379,806

claims. or. 315-168) This invention relates to alarm systems sensitive to sonic vibrations, and has application to the protection of bodies of water, such as unattended swimming pools, fish ponds, and the like, for indicating the falling of a body into the water. It is useful as a lifesaving device to protect people who accidentally fall into such bodies of water, and is also useful to indicate trespassing.

This application is a division of application Serial No. 142,248, filed February 3, 1950, in the names of Donald M. McCoy and Justus M. D. R. Armstrong.

A broad object of the invention is to provide a sensitive and reliable system for indicating the presence of sonic waves.

Another object is to provide a practicable alarm or indicating system capable of responding to the disturbance of water produced by a foreign body falling into it.

Another object is to provide an indicating system for protecting a swimming pool or the like that can be substantially concealed and can be completely controlled at a point remote from the body of water that is protected.

It has heretofore been proposed to produce an alarm in response to the falling of an object into the water by a.

switch device floating on the water that closed an alarm circuit when agitated by surface waves. Such devices have the disadvantage that they are readily visible and invite tampering by curious people, and that they must 'be manually reset after each operation.

The present invention is based on a different approach to the problem. Instead of utilizing the surface waves, it utilizes the pressure or-sonic waves that are transmitted hydrophone that is submerged in the water so that it is concealed, 'or at least is inconspicuous and thereby not sub- The electrical waves are conducted by a suitable, tran'smissionline to a control box containing an amplifier and a detector; the control box being locatedat any convenient point. .In normal use the hydrophone ne'ed'not be accessible, the reset operation being performed at the control box.

The electrical system of the inventionv includes certain novel features that are desirable, if not essential, to reliable operation, since they'reduce the possibility of false alarms by extraneous potentials, as distinct from poten- 'tials originating in the hydrophone These features in- ,clude' an exclusion filter for excluding from the amplifier 1 stray alternatingcurrent potentials induced by commercial electrical power lines in the vicinity, and a detector circuit ;-that is immuneto transient circuits developed by'the reset switching operation, This latter feature may have general application to variou s trip circuits in addition to its application to a sonic wave detection circuit. I

-The manner in which the aforementioned objects of the T 2,832,915 Patented Apr. 29, 1958 2 with the invention applied to the protection of a swimming pool; and 1 Fig. 2 is a schematic diagram showing the electrical circuit involved in the system of Fig. l.

Referring first to Fig. 1, there is disclosed a swimming pool 10 located a short distance from a building 11. The protective system in accordance with the invention comprises a hydrophone 13 that is submerged in the .water of the pool 10 and is connected by an electrical cable 14 to a control box 15 located in the building 11.. The control box 15 has connected thereto, by connecting lines 16 and 17 respectively, an. alarm bell 18. located within the building 11, and'an alarm bell 19 located ex-teriorly of the building. These alarm bells 18 and 19 may, of course, be located at any desirable point, depending upon the particular circumstances.

The hydrophone 13 responds to sonic vibrations in the pool 10 to produce electrical waves of corresponding frequency which are transmitted through the cable 14. to the control box 15 where they are amplified, detected, and cause to close a relay that completes an energizing circuit to the alarm bells 18 and 19. The electrical circuit will now be described with reference to the circuit diagram of Fig. 2, in which parts corresponding to those in Fig. 1 bear the same reference numbers.

Referring to Fig. 2, the hydrophone 13 may be of any known type, such as a crystal or magnetic pick-up. It should be capable of handling a relatively wide range of frequencies since the pressure waves produced in the water by objects falling thereiuto are not confined to any particular frequency range. They may fall within the audible range, or they may extend into the supersonic range. The hydrophone l3 converts these sonic vibrations into alternating electrical currents orelectrical waves of corresponding frequencies which are transmitted over the cable 14. Since the electrical waves may sometimes be relatively weak, it is desirable to shield the wires conducting them from stray potentials, and if the cable 14 is of appreciable length it should be shielded and may be of the coaxial conductor type.

At the control box 15 the electrical waves transmitted over the cable 14 pass through a filter 21 into an amplifier 22, which amplifier is preferably of the vacuum tube type that builds -thefr'elativ'ely weak waves received over the cable 14 up to a substantialamplitude. The amplified wavesare then detected in a detection circuit 23 and caused to energize a relay 24 which closes a'circuit tothe alarm bells 18 and 19. V

v The filter 21 is of a conventional known type which has a high attenuation for the particular frequency to mercial power supply in' the neighborhood, whichis usually 60 cycles per second. The filter may not always be necessary, but it is frequently found that stray potentials will be induced in the hydrophone 13 or in the cable 14 that may be of substantial magnitude compared to the signal currents generated by the hydrophone. By

virtue of the fact that the filter 21 is tuned to the power frequency, it excludes extraneous currents ofthat frequency without materially reducing the energycontent of the waves produced by the hydrophone 13 in response to a shock in the water. 1

The detector circuit 23 comprises a thyratron tube 26 having a cathode 27, control grid 28 and anode 29, and containing a gas that ionizes to produce a relatively large current flow. It is'a well-known characteristicof such tubes that with a positive potential on the anode 29, the tube can be maintained in non-conducting condition by 24 and a normally closed push button switch 31 with the positive lead 32 of a power pack The anode circuit is. opened to extinguish the tube by momentarily actuating the push button switch 31.

The amplified waves from the amplifier 22 are applied through a condenser 35 and a rectifier 36 to the grid 28,

i the rectifier 36 passing the positive half waves and blocking the negative half waves, to drive the grid 28 positive. A small condenser 37 is connected between the grid 28 and ground to maintain the positive potential on the grid between pulses.

To make possible the application of a biasing potential to the grid 28, the rectifier 36 is of a type having a high but finite reverse resistance, so that when the amplifier 22 is not applying positive pulses through the rectifier 36, a negative charge on the grid 28 can leak off through the rectifier 36 and through a grid leak resistor 39 to ground. The cathode 27 is connected to ground through a biasing resistor 40 which is bridged by a relatively large condenser 41. A bleed resistor 42 is connected between the positive lead 32 from the power supply and the cathode 27 to provide a constant minimum current through the biasing resistor 40 at all times that is sufiicient to negatively bias the grid 28 slightly beyond the potential at which the tube 26 fires. After the tube has fired, the current flowing through the biasing resistor 40 from the bleed resistor 42 is augmented by the cathode current of tube 26 which may be substantially greater. Hence, after the tube has fired, the biasing potential on the grid 28 is made substantially more negative for the purpose of increasing the immunity of the tube to reignition when'the push button 31 is opened and closed.

Of course when the switch 31' is opened, the cathode current is immediately broken, but the relatively large condenser 41 thereafter discharges through the resistor the brief interval that the switch 31 is open. Hence when the switch 31 recloses to again apply the anode potential to the anode 29, the high negative bias is still being maintained on the grid 28 so that it will not respond to any transient potential that might be applied to it. Such 9 a transient potential could result from the opening or closing of the switch 31, sincev a relatively, slight transient produced in an early stage'of the amplifier 22 could be built up to a substantial magnitude in the output of the amplifier.

The power supply 33, as shown, is of a conventional type adapted to be connected to an ordinary 115-volt lighting circuit 50. It supplies theusual high potential to the conductor 32 which delivers it to the anode 29 of the tube 26 and to the amplifier 22. It also supplies a low potential over conductor 53 to energizethe bells 18 and 19 and the cathode heaters of the tube 26 and the tubes in the amplifier 22. A pilot lamp 59 indicates when the system is energized for operation.

The power pack 33 is preferably connected to the energizing line through a snap action double-throw switch 54 which in one position connects the power main 50 to the power pack, and in the other position energizes one or more signal lamps 61 and 62 which may be suitably located as a warning that the'protective system is not in operation.

Although for the purpose of explaining the invention,

skilledin the art, and I do not desire to be limited to the exact details shown and described.

I claim: 7

1. An alarm circuit for producing an alarm in response to electrical waves of predetermined magnitude comprising: an input terminal to which said electrical waves are applied; an electronic tube of the gas discharge type having a cathode, control grid and anode; a source of direct anode potential having positive and negative terminals; load means and switch means in series connecting said terminal to said anode; a cathode resistor connecting said negative terminal to said cathode; means forming a grid biasing path between said control. grid and said negative terminal; means for applying positive pulses from said input terminal to said grid; said cathode resistor being of such magnitude as to" develop a negative 7 biasing potential on said grid exceeding said positive pulses from said input terminal when said tube is conducting current; and a condenser bridging said cathode resistorand maintaining flow of biasing current therethrough during momentary opening and closing of said switch means, whereby said tube is temporarily rendered abnormally insensitive at the time of reclosure of said switching means. 7 7

2. Apparatus according to claim 1 including means for maintaining a bias on said grid independent of the cathode current, and of such magnitude as to leader said tube normally non-conductive but rendering it conductive in response to said positivepulses from said input'terminal.

3. Apparatus according to claim 2 in which said means for applying positive pulses from said input terminal to said grid includes a rectifier in series between said input terminal and said grid, said rectifier having a high but finite inverse resistance; and said grid biasing path comprising a leak resistor in series with said rectifier between said grid and said negative terminal.

4. In combination, an electronic tube of the gas discharge type having a cathode, anode and control grid, a source of anode potential having positive and negative terminals meansincluding a switch connecting said positive terminal to said anode; a cathode biasing resistor connecting said cathode tofthe negative terminal of said anode supply for producing a potential drop in response 40 and maintains a substantial potential thereacross for to flow of cathode current therethrough; means for applying a triggering potential to said grid to institute a discharge in said tube; and a condenser of substantial capacity in shunt to said cathode resistonfor maintaining said potential drop thereacross during momentary opening of said switch. 7

nal anode supply throughsaid rectifier to said grid, said rectifier having a high but finite inverse resistance.

References Cited in the file of this patent 1 UNITED STATES PATENTS 2,275,027. Bell Mar. 3, 1942 2,288,554 Smith June 30, 1942' 2,310,105 Michel Feb. 2, 1943 2,398,432 Livermore Apr. 16, 1946 2,428,290 Peck Sept. 30, 1947 2,441,677 Stallsmith May 18, 1948 2,442,430, OBrien June 1-, 1948 2,457,125 Chatterjea et al. Dec. 28, 1948 2,457,176 Preisman Dec. 28,1948 2,462,945 Carlson Mar. 1, 1949 2,470,303 Greenough May17, 1949 2,509,345 Howell et al. May 30, 1950 2,607,023

Coughlin Aug. 1 2, 1952 

